[Amps] Alpha 77Dx -Adding Grounded Grid Filament Choke

R.Measures r at somis.org
Sat Mar 5 11:14:28 EST 2005


On Mar 5, 2005, at 7:57 AM, Paul Hewitt wrote:

> Hi Rich
> For the 8877 at HF you can save the filament choke, a simple gas 
> discharge
> spark gap at the cathode will fire long before an internal arc to 
> cathode
> will jump to the filament.

What is the peak-I rating of the gas-discharge device, the V-drop 
across it at the peak-I rating, and what is its cost?

>  The discharge tube insures the cathode stays no
> more than the gaps voltage rating above ground potential during a HV 
> arc to
> cathode

The problem is not a HV arc to cathode, the problem is a +HV arc to 
chassis ground -- which in turn tries to elevate the cathode to the neg 
amount of HV.  Since this is typically several neg. kV, and the max-V 
rating between the heater and the cathode is c, 150v, there's trouble 
in river city.

> and will fire long before a "glitch diode" will turn on and conduct.

Glitch diodes are thoroughly on at 1v per junction.

> Eimac recommends this on the 8877's bigger brothers. Un-grounding the
> filament secondary is a good idea also, no sense having the filament 
> looking
> like a lower impedance path to ground than the spark gap !!
> Cheers
>
> PAUL HEWITT
> WD7S PRODUCTIONS
> QRO HOMEBREW COMPONENTS
> http://home.earthlink.net/~wd7s
>
>
>
> ----- Original Message -----
> From: "R.Measures" <r at somis.org>
> To: "Paul Christensen" <w9ac at arrl.net>
> Cc: "AMPS" <amps at contesting.com>
> Sent: Saturday, March 05, 2005 12:55 AM
> Subject: Re: [Amps] Alpha 77Dx -Adding Grounded Grid Filament Choke
>
>
>>
>> On Mar 4, 2005, at 6:30 AM, Paul Christensen wrote:
>>
>>> Whoops - my mistake.  The 77Dx's 8877 does not tie the cathode and
>>> filament
>>> together.  One side of the filament is grounded.  Would it still be
>>> advisable to add the common-mode filament choke anyway?
>>
>> Not if you have a free supply of 8877s.
>> -  I would add a bifilar RFC, unground the filament/heater winding, 
>> and
>> wire the cathodes to one side of the heater.
>>>
>>> -Paul, W9AC
>>>
>
>
>
>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org



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